1. Field of the Invention
This invention relates generally to an improved process for depositing an adherent metal layer on a substrate having an electrophoretically deposited coating thereon and the article produced thereby. More particularly, this invention relates to metal core printed circuits and a process for producing same.
2. Description of the Prior Art
Metal core substrates used for printed circuits have an insulating layer to insulate individual printed circuit conductors from the metal core. Among the advantages of such metal core substrates are temperature equalization, ground plane shielding, dimensional stability, elimination of warp and high structural strength.
Metal core boards have been used when a heat conductive core is needed to carry heat out of a circuit package. Oversized holes are drilled in a metal core such as aluminum; copper-clad epoxy prepreg is laminated on each side, taking care to fill in the holes completely with epoxy; the holes are redrilled to size; and then the circuit is fabricated by conventional processing including plating through connections by means of the holes in the epoxy plugs.
For example, to produce a plated through hole with a finished diameter of 1 mm, a 1.15 mm diameter hole must be drilled in the epoxy plug to allow for copper plating to a minimum thickness of, e.g., 0.05 mm on the hole walls. Also, an epoxy ring having a minimum thickness of 0.25 mm must remain in the hole to prevent the entire epoxy plug from being torn out of the hole during the second drilling operation. In addition, allowance must be made for cumulative registration errors in the first and second drilling operations by adding another 0.08 to 0.13 mm thickness to the diameter of the original hole to be drilled in the metal substrate. Thus, to produce a finished hole diameter of 1 mm, the original hole in the metal substrate must be drilled to a diameter of 1.73 mm to 1.78 mm. When holes are drilled on 2.54 mm centers, the double drilling operation leaves a web having a width of 0.76 mm to 0.81 mm. The hole to hole distance cannot be made smaller than 2.54 mm because the first hole must be so much larger than the finished hole. This makes it difficult to increase the component packaging density.
The epoxy plugs sometimes are formed by laminating a sheet of copper foil with one or more sheets of glass cloth impregnated with epoxy resin (epoxy prepregs) to one side of the metal substrate. The epoxy resin flows through the holes in the substrate forming the plugs. Then a sheet of copper foil and epoxy prepregs are laminated to the second side of the metal substrate. Lamination employed in this method is a two stage process. If lamination of both sides of the metal substrate occurred simultaneously instead of successively, air would easily be trapped in holes in the metal substrates when both ends of the holes were covered with impervious layers. With the two stage lamination process, it is easier for air bubbles to escape and for a uniform plug of epoxy to fill the holes. In addition, with the two stage lamination process, holes can be inspected after the first stage is completed to ensure uniform filling occurs before further processing continues.
This two stage lamination process has disadvantages. Manufacturing yields are poor. Also, the costs associated with high flow resins, double drilling, double laminating and the problems associated with maintaining tolerances during the double drilling steps are about seven times the cost of a conventional printed circuit board. Therefore, this type of metal core printed circuit board has not found wide acceptance, but has been used for temperature equalization in avionics, where conventional cooling systems are too bulky and ineffective.
Metal core printed circuits also have been manufactured in large quantities utilizing steel substrates having a thickness of 0.5 to 1 mm. Holes for through connections are punched in the steel substrates. Then, the steel substrates are coated by a fluidized bed powder coating process which deposits an insulating layer of expoxy resin in powder form on the substrates and on the walls of the holes therein. However, when the epoxy resin powder on the steel substrate is fused, as is required to cure the coating, the epoxy flows slightly. This creates a thin coating at the top and bottom rims of the holes, and a thick ring of epoxy resin in the center of the holes. To produce an adequate coating thickness at the top and bottom rims of the holes, it is necessary to deposit resin to a thickness of 0.35 mm or more on the surface and in the center of the holes. Thus, 2 mm holes must be formed on 2.54 mm pitch in order to achieve the standard 1 mm hole required for Dual-In-line-Packages. Because only 0.5 mm of the steel substrate remains between the holes, the surface between the holes is rounded, not flat, and it is difficult to print on such surface. This limited the usefulness of the powder coating technique with respect to high density circuit patterns.
Electrophoretically deposited polymers have been widely used as the primer coating on automobile bodies and a variety of appliances. Japanese Patent Publication No. 39076 of 1981 suggests that metal substrates for printed circuits can be insulated with electrophoretically deposited coatings. The coatings flow however, when they are cured, and, as in the case of epoxy powder fluidized bed coatings, this produces a thin coating at the top and bottom rims of the holes and an excessively thick coating in the center. The process therefore has not been successful in making a commercially useful product.
Japanese Patent Publication No. 51-49468 of 1976; Japanese Patent Publication No. 53-111470 of 1978; Japanese Patent Publication No. 55-24716 of 1980; and U.S. Pat. No. 4,321,290 disclose that a truly cylindrical surface in the holes can be achieved by first coating the surfaces of a metal substrate with an insulating layer of uniform thickness before producing holes for through connections; after the holes are made, the metal walls of the holes are etched back creating an overhang or lip of the insulating layer at the top and bottom of the holes walls. Then, a resin is electrophoretically deposited on the metal walls of the holes creating a uniform film of resin on the hole walls without producing thinning of the coating at the top and bottom rims of the holes, or a thick ring at the center.
In practice, it has been found that the known resin systems for electrophoretic depositions are not a suitable base for the metallized through hole walls required in the printed circuit art. Despite large investments of manpower and research efforts by major manufacturers of electronic equipment for the computer and telecommunications industries, attempts to practice these inventions therefore were unsuccessful. It was found that copper deposits would not adhere to the hole walls and further that the electrical insulating properties are inadequate. For these reasons, the teachings of U.S. Pat. No. 4,321,290, Japanese Patent Publication No. 51-49468 of 1976, Japanese Patent Publication No. 53-111470 of 1978, and Japanese Patent Publication No. 55-24716 of 1980 have not found commercial application.